Rhenium and Technetium Complexes with Pentadentate Thiocarbamoylbenzamidines: Steps toward Bioconjugation

Thiocarbamoylbenzimidoyl chlorides, PhC(Cl)═N-(C═S)-NR(1)R(2), react with 2-(iminodiacetic acid)benzylamine under formation of the potentially pentadentate ligands H3L (R(1), R(2) = Et) and H3L-COOEt (R(1) = Me, R(2) = C6H4-4-COOEt) in high yields. Hydrolysis of H3L-COOEt in NaOH/MeOH gives quantitatively another benzamidine ligand H3L-COOH. The novel ligands readily react with (NBu4)[MOCl4] (M = Re, Tc) under formation of stable complexes with the general composition [MO(L)], in which they are triply deprotonated and fully occupy the remaining five coordination positions of the {MO}(3+) cores. In a "proof-of-principle" reaction for possible bioconjugations, the complex [ReO(L-COOH)] has been labeled with triglycine ethyl ester in high yields.